1. Field of the Invention
The present invention is in the field of gas analyzers and specifically relates to a sample chamber for use in a gas analyzer of the type known as an NDIR (nondispersive infrared) analyzer.
2. The Prior Art
The NDIR technique has long been considered one of the best methods for gas measurement. In addition to being highly specific, the NDIR-gas analyzers are also very sensitive, stable, reliable, and easy to maintain. The major drawback of the NDIR gas measurement technique has been its complicated and expensive implementation.
An NDIR gas analyzer typically includes an infrared source, a motor-driven mechanical chopper to modulate the source so that synchronous detection can be used, a pump to push or pull gas through a sample chamber, a bandpass filter, a sensitive infrared detector plus expensive infrared optics and windows to focus the infrared energy from the source onto the detector. Thus, despite the fact that the NDIR gas measurement technique is one of the best, it has not found wide application because of its complexity and high cost of implementation.
The present invention significantly simplifies the implementation of the NDIR gas measurement technique, and this simplification results in a concomitant reduction in cost, thereby opening dozens of applications for the NDIR technique that were heretofore considered impractical because of cost or complexity.
For example, the sample chamber of the present invention is at the heart of a much faster and sensitive carbon dioxide detector for use in sensing fires, (U.S. Pat. No. 5,053,754 issued Oct. 1, 1991 to the present applicant), and is also at the heart of a ventilation controller or VENTOSTAT (the thermostat of ventilation as described in U.S. patent application No. 07/611,630 filed Jun. 6, 1991 for VENTILATION CONTROLLER by the present inventor), which is highly useful in combatting indoor air pollution by monitoring the concentration of carbon dioxide in the indoor air and bringing in fresh air when the carbon dioxide concentration is excessive.
The present invention for a simplified gas sample chamber provides a novel approach for reducing the complexity of NDIR gas measurement systems by eliminating the need for: expensive optics, mechanical choppers, and a pump for pulling or pushing the gas into the sample chamber. In addition, the sample chamber of the present invention provides a long effective pathlength which increases the detection sensitivity.
In U.S. Pat. No. 4,709,150 issued Nov. 24, 1987 to Burough et al., there is described a gas sample chamber that consists of a tube composed of a porous material such as plastic or a sintered metal. Burough et al. teach that the pore size should be from 0.3 to 100 microns. There is no teaching or suggestion of using the walls of the porous tube as reflective radiation-guiding elements. Perhaps for this reason, the problem of condensation of the gas into droplets on the interior of the sample cell is not addressed.
Burough et al. do not teach or suggest multiple reflections from a specularly-reflective surface. This seriously affects the performance of their system. Without taking advantage of the radiation-collecting ability of the sample chamber, the system of Burough et al. has much poorer radiation collecting ability, leading to a lower signal-to-noise ratio. Furthermore, the system of Burough et al. does not have provision for a long pathlength, and hence the sensitivity of his system suffers in comparison with the present invention.
With regard to the diffusion of gas into the chamber of Burough et al., as compared to the present invention, it is noted that the porous material used in the sample chamber of Burough et al. is several hundreds of microns thick. In contrast, in the present invention, the diffusion into the sample chamber takes place through a semi-permeable membrane which is on the order of 25 to 50 microns thick. Accordingly, it takes much longer for the gas, or changes in the concentration in the gas, to diffuse into the chamber of Burough et al., as compared with the present invention. This greatly lengthens the response time of the chamber of Burough et al., thereby making it a poor choice for a fire detecting sensor, whereas the chamber of the present invention responds very rapidly to changes in the carbon dioxide concentration, and laboratory tests have demonstrated that the sample chamber of the present invention has an extremely fast response time which is highly desirable in a fire detector.
In Japanese Patent Publication (Kokai) No. 59-173734(A), Miyazaki describes an infrared ray gas analysis meter in which radiation proceeds in parallel along a sample cell and a reference cell. These cells have the form of a helical tube.
Miyazaki's system, as disclosed in his patent, falls under the category of a conventional NDIR gas measurement system. Were it not for the fact that the incident radiation undergoes multiple reflections inside both the sample and reference cells, there would be no difference from a conventional NDIR system, and consequently no advantage at all. Miyazaki's design still calls for a mechanical chopper, pumps to direct gases through both the sample and reference cells, and two detectors. Thus, when these factors are taken into consideration, Miyazaki's invention does not come close in simplicity and efficiency to the present invention.
In Japanese Patent Publication (Kokai) No. 63-298031(A), Fujimura discloses the use of a filter, which is required in his invention since the source of radiation and the detectors used in his system reside inside the sample chamber and are thus subject to contamination by the sample.
In U.S. Pat. No. 4,499,379 issued Feb. 12, 1985 to Miyatake et al. and in U.S. Pat. No. 4,501,968 issued Feb. 26, 1985 to Ebi et al., there is described a gas analyzer having a heated sample gas container for containing a sample gas at a temperature at which the component whose concentration is to be determined will emit infrared radiation of a characteristic wavelength. This gas analyzer works on an emission principle and is not a nondispersive infrared absorption analyzer. A heater in the wall of the sample cell heats the sample gas to temperatures of at least 100.degree. C. to cause the gas to radiate infrared. This is said to increase the radiation from a sample of the gas while decreasing the background radiation relative to the radiation from the gas. The internal surface of the sample cell is said to be a mirror surface, but the patents give no reason for this. Since the gas itself is the source of the radiation, which is isotropic, it does not appear that the walls of the chamber would serve to guide the radiation in any useful way.
In U.S. Pat. No. 3,966,439 issued Jun. 29, 1976 to Vennos, there is described a fluid sampling device that includes a pump and that is used for accumulating a sample of particles found in the air, in factories, power plants, mines, etc.
Vennos is not concerned with passing infrared radiation through a gaseous sample to determine its concentration, and thus the filtering system of Vennos is from a nonanalogous art.
Likewise, in U.S. Pat. No. 4,947,578 issued Aug. 14, 1990 to Anderson et al., there is described a controlled release system for an insect attractant. In this patent the attractant vapor is allowed to diffuse through a membrane. Because the pore size is determined by the desired release rate, the use of the membrane by Anderson et al. is not analogous to that of the present invention.